During the 1930s and 1940s, other enzymes such as trypsin and trypsinogen, chymotrysin and chymotrypsinogen, pepsin and pepsinogen, were crystallized by J.H. Northrup, R.M. Herriott, and M.

In the following experiments, cytochrome c was modified by splitting off a great part of the protein portion of the molecule with pepsin. The resulting substance was then isolated and studied.